docs(router): make getCurrentNavigation() jsdoc more explicit (#41417)

Add additional clarification to the documentation for
`getCurrentNavigation`.

PR Close #41417

packages/router/src/router.ts

@@ -1027,7 +1027,10 @@ export class Router {
     return this.serializeUrl(this.currentUrlTree);
   }
 
-  /** The current Navigation object if one exists */
+  /**
+   * Returns the current `Navigation` object when the router is navigating,
+   * and `null` when idle.
+   */
   getCurrentNavigation(): Navigation|null {
     return this.currentNavigation;
   }